1. Field of the Invention
The present invention relates to a semiconductor laser module for a wavelength divisional multiplex (WDM) system. More particularly, the invention relates to a semiconductor laser module capable of accurately materializing a desired oscillation wavelength and fit for WDM system.
2. Description of the Related Art
A semiconductor laser module has a semiconductor optical amplifier in combination with an optical fiber for propagating the laser light generated by the semiconductor optical amplifier efficiently depending on the application. Further, the semiconductor optical amplifier essentially consists of a light-emitting element as a light source, and an optical resonator including a pair of reflectors for mutually reflecting the light emitted from the light-emitting element.
FIG. 4 is a diagram illustrating the structure of a typical semiconductor laser module. As shown in FIG. 4, a semiconductor laser 35 is packaged on a base 30 via a sub-mount 31 in the ordinary semiconductor laser module. One side of the base 30 has a perpendicular edge face and an optical system 32 is attached thereto as shown in FIG. 4. Further, a ferrule 21 holding the optical fiber 20 is securely passed through a through-hole provided in the side wall of the package 10. The edge face of the optical fiber 20 is so arranged as to face the semiconductor laser 35 via the optical system 32, so that the light emitted from the semiconductor laser 35 is efficiently coupled to the optical fiber 20.
Further, a series of functional members mentioned above is normally housed in the package 10 in such a state that the functional members are orderly mounted on temperature control elements such as Peltier effect elements 34. The functional members are also made to keep the operating temperature constant under feedback control using a temperature detection element (not shown) that is mounted on the sub-mount 31 together with the semiconductor laser 35.
With the progress of information processing technology now, it is clearly demanded that the transmission density be improved even in the optical information communication field using semiconductor laser modules. The reason for this is that an amount of information to be transmitted has increased to an extremely greater extent in addition to expansion of a field of utilization.
In order to satisfy the above demand, a WDM system is now in progress. In other words, the WDM system allows the transmission speed to be practically improved by superposing a plurality of optical signals having different wavelengths and transmitting the optical signals thus superposed through one light transmission line.
FIG. 5 is a conceptual diagram illustrating a WDM system configuration.
A system shown in FIG. 5 includes a plurality of light sources 101, a mixer 103, a branching device 104 and a plurality of receivers 105. The plurality of light sources 101 each have discrete wavelengths xcex1, xcex2 . . . xcexn. The mixer 103 injects the light signals emitted from the light sources 101 into a light transmission line 102. The branching device 104 separates the light signals propagated through the light transmission line 102 on a wavelength basis. The plurality of receivers 105 receive the respective light signals thus separated by the branching device 104.
As the above, The light sources 101 in the WDM system each have the discrete wavelengths. In the case of a 1.55 xcexcm band, for example, it has been standardized to use 32 wavelengths increasing at 0.8 nm intervals from 1535.8 nm as shown in the following table 1.
When it is attempted to obtain the plurality of oscillation wavelengths at the narrow intervals mentioned above, the light sources are required to have monochromatism and stability. Hence, no satisfactory characteristics are available from a method of directly utilizing the oscillation wavelength of a Fabry-Pxc3xa9rot type semiconductor laser with both edge faces of a semiconductor chip as mirrors of the resonator. Consequently, it has been proposed to obtain desired characteristics by incorporating a diffraction grating into the semiconductor laser element to make a DFB(distribute feedback) or DBR(distributed Bragg reflector) laser.
In the DFB or DBR laser, the oscillation wavelength is determined by the diffraction wavelength of the diffraction grating formed within the semiconductor laser and the gain of the active layer. In other words, as shown in FIG. 6, the reflection spectrum A of the diffraction grating, the longitudinal mode B of the optical resonator including the diffraction grating, and the gain C of the semiconductor optical amplifier have respectively different characteristics. Accordingly, laser oscillation is produced at a wavelength where the product of these characteristics is maximized.
Further, by sufficiently sharpening the reflection spectrum A of the diffraction wavelength of the diffraction grating, the diffraction wavelength becomes actually a substantial oscillation wavelength as shown in FIG. 7. Today, the oscillation spectrum width of the DFB laser has reached GHz order and this can be utilized satisfactorily for the WDM system in view of sharpening the spectrum.
As stated above, the characteristics of the diffraction grating that substantially determine the oscillation wavelength are determined in the laser manufacturing process. It is consequently hard to manufacture semiconductor lasers having specific oscillation wavelengths at narrow intervals conforming to the standards shown in Table 1.
As the diffraction grating is incorporated in the semiconductor laser, the diffraction grating will be directly affected by the temperature characteristics of the semiconductor, and the oscillation wavelength may vary with the environmental temperature change and the heat generation of the semiconductor laser itself. Although the oscillation wavelength changes slightly, it cannot be disregarded for the WDM system using the plurality of light sources with different wavelengths at 0.8 nm intervals.
An object of the present invention is to provide a novel semiconductor laser module having different oscillation wavelengths at slight intervals.
A semiconductor laser module according to the invention comprises a light-emitting element emitting light, a package, an optical resonator and an optical fiber. The package houses the light-emitting element therein. The optical resonator has a pair of opposed reflectors for reflecting the light emitted from the light-emitting element. The optical fiber leads out laser light generated in the light-emitting element through the optical resonator. The optical fiber has a diffraction grating disposed close to an end portion of the optical fiber. One of the reflectors of the optical resonator is a reflective film formed on one edge face of the light-emitting element and the other reflector thereof is the diffraction grating disposed close to the end portion of the optical fiber.
The above-mentioned semiconductor laser module preferably comprises an optical connector attached to the end portion of the optical fiber, wherein the optical fiber is connected to the package via the optical connector. In the semiconductor laser module, it is advantageous that the optical connector resiliently supports the optical fiber therein and wherein when the optical connector is attached to the package, the optical fiber is abutted against the package so as to be automatically positioned.
A semiconductor laser module according to the invention features that an optical fiber is provided with part of an optical resonator, that is a diffraction grating, and by properly selecting the optical fiber provided with a diffraction grating, the optical fiber thus selected is allowed to incorporate an optical amplifier contained in a package.
The structure of a semiconductor optical amplifier according to the invention is basically similar to that of a Fabry-Pxc3xa9rot type semiconductor laser element. More specifically, its reflectance is lowered as much as possible by forming an extremely low reflective film on the emission-side edge face of the semiconductor optical amplifier. As any optical resonator is not formed in a single semiconductor optical amplifier, no laser oscillation is generated in that single body.
The oscillation wavelength in the semiconductor laser module is generally determined by the diffraction wavelength spectrum of the diffraction grating forming the optical resonator and the gain characteristics within the semiconductor optical amplifier. In this case, the semiconductor laser module can be oscillated with this diffraction wavelength by sufficiently sharpening the diffraction spectrum. Moreover, half width of oscillation wavelength is almost similar to what is available from the DFB or DBR laser.
Further, in the above-mentioned semiconductor laser module according to the invention, fiber grating (hereinafter called the xe2x80x9cFGxe2x80x9d) formed in the optical fiber is used as the diffraction grating. The diffraction wavelength of the FG can be determined entirely differently from the manufacturing process of semiconductor optical amplifiers. The well controlled FG can be produced superior to that in the case where diffraction grating to be formed in the semiconductor optical amplifier. The FG is also less affected by the ambient temperature during the operation. Moreover, the optical fiber that is usually made of glass will never generate heat itself. Therefore, the semiconductor laser module is allowed to easily select an oscillation wavelength.
Moreover, in the above-mentioned semiconductor laser module according to the invention, it is preferable that an optical connector is fitted to the optical fiber formed with the FG and is attached to the package of the semiconductor optical amplifier. With this arrangement, the FG becomes easily replaceable, so that a semiconductor laser module having a desired oscillation wavelength can be supplied.
Although a specific example of the invention will now be described with reference to the drawings, the description thereof refers to only an embodiment of the invention and never limits the technical scope thereof.